With steadily-increasing access to the Internet, the role of communication networks is approaching a great point of change. The core of conventional communications traffic is voice telephone communications, and the vital role of communication networks is to build up a voice communication network covering the global efficiently to provide communication services. A SONET ring is an example.
FIG. 18 shows a conventional SONET ring adopting a 2-fiber/4-fiber BLSR. In the SONET ring 100 shown in the figure, communication devices 101a to 104a are arranged in a ring configuration and interconnected through two or four optical fibers.
The SONET ring 100 is also connected with routers 101b to 104b, which are used for transmitting data traffic. More specifically, the communication device 101a is connected with the router 101b, the communication device 102a is connected with the router 102b, the communication device 103a is connected with the routers 103b-1 to 103b-3, and the communication device 104a is connected with the router 104b. 
This enables any pairs of routers to transmit data traffic between them through the SONET ring 100. That is, as shown in FIG. 18, the routers 101b and 102b are able to transmit data traffic between them through the communication devices 101a and 102a. 
Likewise, the routers 103b-1 and 102b are able to transmit data traffic through the communication devices 103a and 102a. The routers 101b and 103b-2 are able to transmit data traffic through communication devices 101a, 104a, and 103a, and the routers 103b-3 and 104b are able to transmit data traffic through the communication devices 103a and 104a. Note that any pairs of routers are connected with each other through SOENT cross-connect.
Between the communication devices of the above-described SONET ring 100, there are set many transmission channels along transmission directions (EW and WE directions indicated by arrows), through which frames are transmitted. Further, bidirectional line switched rings (BLSRs) have a redundant channel structure consisting of working channels and backup channels.
For example, a 2F-BLSR with Optical Carrier-Level 48 (or OC-48 level) includes 48 channels in both the EW direction and the WE direction. Among these, bidirectional channels #1 to #24 (ch #1 to ch #24) are set as working channels, and channels #25 to #48 are set as backup channels.
More specifically, the backup channels for the working channels #1 to #24 in the WE direction correspond to the backup channels #25 to #48 in the EW direction opposite to the WE direction. The backup channels for the working channels #1 to #24 in the EW direction correspond to the backup channels #25 to #48 in the WE direction opposite to the EW direction.
In the above-described channel configuration, when there is no failure, backup channels that correspond to half of the total number of channels are kept in reserve to assure reliability of communications incase of a failure. For instance, in the case of the above-described OC-48 level, 2.4 Gbit/s (Gbps) can be transmitted when all channels are used, and half the bandwidth (i.e., 1.2 Gbps) is kept in reserve in case of a failure.
However, the above-described SONET ring requires high reliability because it is primarily used for transmitting voice traffic. Particularly, it cannot be said that networks adopting BLSR are using bandwidth efficiently when there is no failure. With an increase in the number of Internet users, the amount of data traffic is increasing exponentially.
In these circumstances, the role of communication networks is changing to efficient transmission of increasing data traffic. Because of such a change, in SONET rings it is becoming necessary to use network resources efficiently for efficient data transmission.